We plan to study the environment of divalent transition metal ions at the active sites of enzymes. We will attempt to identify the nature of the ligands, the liganding geometries and the effect of substrates, inhibitors and allosteric cofactors on the nature of the metal-protein interaction. A systematic study of enzymatically active metalloproteins, such as pyruvate kinase and adenylate kinase, substituted with a variety of transition metal ions will be made. We will take advantage of the fact that contributions of metal d-d electronic transitions to the absorption, circular dichroism and magnetic circular dichroism are very sensitive to the nature of the ligands and the ligand geometry. By carrying out calculations and by using the large body of well established empirical data we will make assignments of the electronic transitions, and through them characterize the metal-ligand interaction.